POLYZOA FROM THE SILURIAN FORMATION. 261 



P. falciforrais in shape, is distinguished by the nearly square cells, with 

 intercellular sulci, and by the fact that the cells are arranged in longi- 

 tudinal lines. 



♦ Position and locality: Not uncommon in the Cincinnati group, near Cincinnati. 

 The specimens described are from the cabinet of Mr. U. P. James. 



Ptilodictya emacerata, Mcholson. 



Plate 25, figs. 5, 5b. 

 Polyzoary consisting of minute, narrow, linear fronds, which branch 

 dichotomously, and have the form of a much flattened, acutely pointed 

 ellipse, in transverse section. Width, one-third of a line ; length of 

 largest specimen observed, two lines. Cells elliptical, their long axis 

 corresponding with that of the branches, about six or seven in the space 

 of one line measured longitudinally. There are four, five, or, rarely, six 

 rows of cells in the frond. When four rows of cells are present, the two 

 central rows are longitudinal, and one row on each side is composed of 

 cells directed in an obliquely ascending manner. When there are five 

 rows, as seems to be most commonly the case, the three central rows are 

 longitudinal, and a row on each side is oblique. When there are six 

 rows of cells, two central ones are longitudinal, and two on each side are 

 oblique. The cell-mouths are much longer than wide, and each row is 

 separated from the next by an elevated line. The lateral margin of the 

 frond on each side forms an obtuse non-celluliferous edge, the width of 

 which is so small that it can not always be detected. A central axis 

 was not clearly determined, but such a structure is doubtless present. 



The only previously recorded species of the genus with which Ptilo- 

 dictya emacerata presents any close resemblance is P.fragilis, Billings, 

 from strata of about the same age in Anticosti (Cat. Sil. Foss. of Anti- ^ 

 costi, p. 9). Our species, however, is distinguished from the latter by its 

 uniformly more minute dimensions, the smaller number of rows of cells 

 in the frond, and the possession in general of no more than a single row 

 of oblique cells on each side. P.fragilis, on the other hand, has a width 

 of from two-thirds of a line to one line, with from eight to ten rows of 

 cells, and two or three rows of oblique margical cells on each side. It is 

 possible our form is only a variety of P. fragilis; but in the absence of 

 figures of the latter, and in the face of the difierences above mentioned, 

 I think it safest, in the meanwhile, to regard P. emacerata as a distinct 

 sj)ecies. 



Position and locality : Cincinnati group, near Cincinnati. The specimens described 

 are from the collection of Mr. U. P. James. 



